Worm gear drives comprise a worm or helical cog driven by a shaft and in mesh with a worm wheel driving an output, typically a shaft or cog. Worm gear drives are speed step down gearboxes and as such increase the torque developed by the motor driving the shaft. Usually, the driving shaft is the motor shaft and the worm is fitted to the shaft or formed on the shaft.
In certain applications the use of a worm gear drive is desirable due to the simplicity of construction and because of the torque able to be delivered because of the gear ratio. There is a certain loss of power due to friction between the worm and the worm wheel. By making certain modifications, this friction can lead to an arrangement where the load cannot back drive the motor due to the losses in the gears. This is known as self locking, i.e., the input shaft can drive the output shaft but the output shaft can not drive the input shaft, without the use of additional components.
This is highly appreciated in certain applications, such as for lifting heavy loads or for security issues such as for doors and windows, especially windows in a vehicle, where to prevent theft it is important that the windows cannot be pulled down to gain access.
To achieve self locking of the gear train, the design of the worm and the worm wheel has to be modified from ideal or optimal from an efficiency viewpoint. The lead angle is made low, that is, the helical thread of the worm has a greater number of turns per unit of length, also the surface texture of the worm and worm wheel is made rough to increase friction between the two gears thus making it harder for the worm wheel to drive the worm. However, both of these measures decrease the efficiency of the gear drive resulting in the need for a more powerful motor to drive the load. This trade-off has been accepted because of the need for preventing back drive.
A typical prior art worm gear drive has a motor driving a motor shaft. A worm fitted to the shaft and in mesh with a worm wheel fixed to an output shaft. The motor shaft is journalled in bearings, typically oil impregnated sintered bushings, and extends between two thrust bearings. Each end of the motor shaft is rounded and each thrust bearing has a hard flat surface which contacts a respective rounded end of the motor shaft to support the shaft against excessive axial movement caused by the reaction forces on the shaft as the worm tries to turn the worm wheel. The rounded end of the shaft makes a point contact with the respective thrust surface, which point lies on the axis of the shaft to minimise frictional losses therebetween. The thrust surfaces also support the shaft against axial movement when the worm wheel tries to drive the worm as when a rotational load is applied to the output shaft. Due to the low lead angle of the worm and the friction of the gear interface, the worm wheel tries to drive the motor shaft axially rather than cause the worm to rotate, thus locking the gears. A small gap, known as end play, is left between the ends of the motor shaft and the thrust bearings under no load conditions. This allows for thermal expansion of the shaft but also means that the shaft only contacts one thrust bearing at a time and slight axial movement of the shaft will occur when the axial forces change direction.
There are other designs of lockable gear drives which allow an efficient gear train and these can use a smaller lighter motor. These designs have a separate braking or clutch mechanism to prevent back drive but these mechanisms are usually complex, prone to failure and add weight and cost to the final assembly. As a separate locking mechanism is required, these drives are not self locking.
Accordingly, there is the desire for a self locking worm gear drive which is simple and efficient.